This application claims priority from European Patent Application 02258208.4, filed Nov. 28, 2002 which is herein incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates generally to a device manufacturing method using a lithographic apparatus and more particularly to computer programs for use in controlling lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning structure, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
To enable imaging of smaller features than is possible with current lithographic projection apparatus, it is proposed to use extreme ultraviolet radiation (EUV), e.g. with a wavelength of 13.5 nm, as the exposure radiation. Such radiation is strongly absorbed by almost all known materials and so a reflective mask is used. However, making a reflective mask for EUV presents its own problems and to achieve an acceptable reflectance, the mask is formed as a distributed Bragg reflector formed by a multilayer of 40 or more layer pairs of, for example, (Mo/Si) or (Mo/Be). The mask pattern is then formed by an overlying patterned absorber layer such as Tantalum (Ta) or Chromium (Cr). The multilayer and absorber layer must be relatively thick, many tens of wavelengths, and this, coupled with the necessity to illuminate the mask obliquely, introduces various errors in the projected image, as compared to an ideal, thin binary mask.
These errors are discussed in various publications. B. S. Bollepalli and F. Cerrina, On the Computation of Reflected Images from Extreme Ultra Violet Masks, SPIE Conference on Emerging Lithographic Technologies III, Santa Clara, Calif., SPIE Volume 3676, 587–597 (March 1999) describes variation of line widths and pattern shifts with angle of incidence of isolated structures and proposes correction by a suitable mask bias. C. G. Krautschik, M. Ito, I. Nishiyama, and K. Otaki, Impact of the EUV mask phase response on the asymmetry of Bossung curves as predicted by rigorous EUV mask simulations, SPIE Conference on Emerging Lithographic Technologies V, Santa Clara, Calif., SPIE Volume 4343 (March 2001) describes asymmetry of the Bossung curve through focus for isolated structures and indicates that different illumination angles experienced by horizontal and vertical lines causes an additional horizontal to vertical CD bias through focus. Again, it is proposed to compensate for these effects through mask-sizing schemes. K Otaki, Asymmetric properties of the Aerial Image in Extreme Ultraviolet Lithography, Jpn. J. Appl. Phys. Vol 39 (2000) pp 6819–6826, describes the influence of asymmetric diffraction when a thick mask is asymmetrically illuminated and notes the asymmetry in the aerial image.
EP-1 251 402-A discloses the idea of deliberately introducing aberrations into a projection lens to compensate for other aberrations already present so as to minimize a merit function. Also disclosed is to compensate for Bossung tilt deriving from deviations from the correct 180° phase shift in a phase-shift mask (PSM). However, the solutions proposed in the prior art do not provide complete solutions and cannot compensate for all mask-induced imaging artifacts.